The primary goal of this double-blind, placebo-controlled, within-subjects functional neuroimaging study is to examine the extent to which the hypnotic zolpidem decreases brain activity in regions of the brain known to process emotional information. Although zolpidem is an effective sleep-aid, its ability to engender anti-anxiety effects is equivocal, yet promising. Zolpidem's activity during tasks that engage anxiety-related processes in the brain will be compared to that of the known anxiolytic drug alprazolam, a positive comparator caffeine, and placebo. A secondary goal of this study is to compare the subjective drug effects, or how individuals feel, following the interventions. These measures will be used to determine the existence of brain-behavior relationships, thus demonstrating that imaging is an important tool for informing us about how drugs produce their effects in the brain.